It has been known for some time that plants produce ethylene by converting methionine through S-adenosylmethionine into l-aminocyclopropane-l-carboxylic acid (ACC) which is then broken down into ethylene, HCN and carbon dioxide. The plant enzyme responsible for the production of ACC is called ACC synthase. Ethylene, a gaseous phytohormone, is believed to be involved in the modulation of a number of plant biochemical pathways affecting such processes as abscission, senescence, flowering, fruit setting, fruit ripening, seed germination, sex expression, root growth, internode elongation, epinasty, and geotropism.
The marketing of cut flowers and ornamental flowering plants is of considerable economic importance to the horticultural industry. The total wholesale market for cut flowers, flowering plants and foliage and bedding plants in 1994 was approximately 1.3 billion dollars. The sale of cut flowers contributed approximately 15% to this total, and the sale of potted flowering plants contributed approximately 23%. The marketing of these products generally involves shipping from the site where the flowers are grown to commercially important markets elsewhere in the country. The handling, packaging and shipping of both cut flowers and ornamental plants places stresses upon them which can result in damage, diminishing their commercial value to the retailer. The stresses placed on plants result in early senescence (aging and browning) which can be due, in part, to the effects of ethylene. Ethylene gas, either produced by the plant itself in response to stress or in the environment causes acceleration of senescence. Plants which look good at harvest with burgeoning flowering buds can look unhealthy and lose many of their buds after transport.
A number of chemicals which limit ethylene-induced damage to cut flowers have been identified. These include silver thiosulfate, 1-methylcyclopropene (MCP) (cf. Plant Growth Regulation, vol. 18, pp. 169-174 (1996), carboxymethoxylamine (also known as aminooxyacetic acid (AOAA)), AVG, rhizobitoxine, and L-trans-2-amino-4-methoxy-3-butenoic acid (MVG). Silver thiosulfate and MCP are believed to inhibit the effect of either internal or external ethylene, while the others are believed to act internally to inhibit the ability of the cut flowers, plants, and fruit to produce ethylene. These compounds (except MCP) are typically applied to plants or plant materials in the form of an aqueous treatment solution. Applications of the treatment solution to potted plants are carried out by spraying it onto the aerial parts of the plants or by including it in the irrigation water which is supplied to their roots. Treatment of cut flowers or greens is typically carried out by immersing the cut ends of the stems in the aqueous solution containing the treating agent immediately after harvest, during transportation or while the floral arrangement is on display, although they might be treated by immersing the whole flowers into a solution or by spraying them. Since MCP is a gas, it cannot readily be applied in aqueous solution, so plants are treated by exposing them to a modified, controlled atmosphere (containing a defined amount of MCP) in an enclosed chamber.
Silver thiosulfate is expensive and cannot be applied to large-acreage, low value crops economically. In addition, it may be toxic to animals, including humans, and for that reason cannot be applied to food or food crops. MCP has not been widely used because of difficulties in production and application and its lack of stability.
The role of ethylene in the ripening of fruit has been recognized in the art for over 40 years. It is known that the rate of production of ethylene in maturing fruit increases while the fruit separates from its pedicel through the formation of a layer of cells with low adhesion known as the abscission layer. If the formation of this layer is completed before the fruit can be picked, the fruit falls to the ground, sustaining injury, which results in a poorer quality. Thus, the prevention of preharvest fruit drop is of significant economic benefit to the grower. In addition, fruit typically has a higher resistance to bruising and penetration injury before or immediately after harvest than after storage for a period of time. The resistance of fruit to penetration is measured with a device called a penetrometer and is reported as fruit firmness. Fruit firmness is also a generally accepted measure of crispness and freshness. Typically at harvest, fruit has a higher fruit firmness than after storage. It is thought that the decrease in fruit firmness over time is, at least indirectly, related to production of ethylene within the fruit.
AVG is a plant growth regulator which inhibits ethylene production. It acts by inhibiting the plant enzyme ACC synthase. However effective AVG is at inhibiting ethylene biosynthesis, there continues to be a need for identifying additional inhibitors of ethylene biosynthesis.